Sorry but some laptops come with windows installation pre-loaded in only one language. Also the only Windows 7 version that got every language is the Windows 7 Ultimate/Integral which has all the language available in it to install it to. So if this is the case, there is no way to change the core language of windows. But you can actually change the outer language, the one Windows is using for anything else only that some minor things will stay german.
To prove that the core language means a lot let's take this into account:
Take two Windows 7 DVDs of the same version say Premium, one in french, one in english. You install them one after another, carefully selecting English as the wanted language for both. Then after this is done you do some updates. BAM! Both don't have the same updates available...